Fix FBString.h inclusion order
Fix FBString.h inclusion order

Summary: I don't see how this header works, as it includes folly/Portable.h (which defines FOLLY_HAS_STRING_VIEW) _after_ it has already used FOLLY_HAS_STRING_VIEW a few lines up. Somehow, that worked with buck1 (really not sure how), but broke buck2.

Reviewed By: ot

Differential Revision: D33511408

fbshipit-source-id: 9e2da2a5c25a02367079254ff78552a5a335b3b1

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly